1. Field of the Invention
The invention generally relates to improvements in thermionic cathodes with carbon-coated surfaces. In particular, the invention provides a cathode in which a gap is present between the carbon coating and the cathode's surface, thereby preventing interaction of carbon and the crystalline emitter material which would otherwise cause damage to the cathode.
2. Background of the Invention
Lanthanum hexaboride (LaB6), cerium hexaboride (CeB6), hafnium carbide (HfC), in sintered or crystalline form, are used as electron sources, or emitters, in various electron-beam tools (e.g. lithographic tools, scanning electron microscopes (SEMs), transmission electron microscopes (TEMs, etc). A typical emitter is tapered, or cone-shaped, with a specified tip or truncation size and cone angle, as shown in FIG. 1A.
The tip (truncation) may be flat or spherical, with diameter from 5 to 100 μm and a cone angle of from 60 to 110 degrees, depending on the application. These cathodes, however, have two built-in disadvantages: Disadvantage 1: at operating temperatures (1650 to 1900 K), emitter material evaporates, and tip size continuously diminishes, which limits the cathode's useful life time. Disadvantage 2: under operating conditions, the electron beam is formed by electrons emitted from both the tip and the cone surface. Electrons emitted from the cone surface constitute up to 65% of the total emission current, but cannot be used in well-focused beams (Ref. 1).
It is thus advantageous to suppress or eliminate material evaporation and electron emission from the cathode cone surface. This may be done, for example, by coating the cone surface with carbon. (Ref. 2). A cross-sectional view of a cathode with a carbon coated cone surface is shown in FIG. 1B. At cathode operating temperatures (e.g. 1650 to 1900 K), the carbon coating's evaporation rate is very low, e.g. ˜1000 times lower than that of LaB6 or CeB6, with a vapor pressure at just 10−10 Torr, which is practically negligible. Hence, the coating does not change its dimensions during the cathode lifetime (about 3000 hrs). In other words, carbon-coated cathodes (e.g. carbon-coated K—LaB6, K—CeB6, and K—HfC) exhibit neither electron emission nor carbon evaporation, and the inherent disadvantages of LaB6 cathodes discussed above are eliminated by the coating
Nevertheless, carbon coatings have disadvantages. Such cathodes have a limited lifetime, caused by both emitter erosion and loss by evaporation, and this loss is caused, in part, by chemical interactions between the carbon coating and the LaB6/CeB6 cathode material. This can be observed in the photograph shown in FIG. 1C, which shows the flat surface of a cone's emitting surface surrounded by the adjacent carbon coating. As can be seen, the edges of the emitting surface of the cone which are in contact with the carbon coating appear to be damaged (e.g. pitted and/or etched). In fact, these areas of the emitting surface are compromised and are no longer capable of efficiently emitting electrons in a focused manner. Thus, in spite of the advantages conferred by the carbon coating, the useful lifetime of the cathode has been attenuated by the contact with the carbon coating.
There is a need in the art to develop alternative ways of extending the lifetime of cathodes. For example, there is a need to develop new cathode coatings that exhibit the positive attributes of carbon coatings, but which do not have the problems associated with carbon coatings.